The present invention relates to a technique for imaging an object to be imaged in synchronization with heartbeats by a radiation tomography apparatus.
One known imaging technique by a radiation tomography apparatus is cardiac-gated imaging in which a heart is an object to be imaged. In the cardiac-gated imaging, a scan is performed targeting a period of time in which a change of the heart is small in each cardiac cycle, and such a scan is repetitively performed a plurality of number of times while shifting the scan region to scan the whole imaging range including the heart. Based on the thus-acquired scan data, image reconstruction is performed for several slices in the imaging range. More particularly, this is achieved by the following procedure, for example.
First, a heart rate monitor such as an electrocardiograph is used to measure the heartbeat waveform of a subject nearly in real time. A reference time for a scan is defined at about 70-% cardiac phase, which is generally equivalent to the middle of diastole of the heart, wherein the cardiac phase is defined as 0% to 100% from one R-wave to the next R-wave in the heartbeat waveform. The reference time is a time serving as reference of a period of time for collecting scan data over a gantry rotation angle i.e., a view angle, required in image reconstruction. Then, a scan is performed on part of the imaging range, which is a scan region, over a certain time span, for example, in a period of time of the order of 0.24 seconds, centering on the reference time. This scan is repetitively performed over a plurality of heartbeats while shifting the scan region for each heartbeat, and the whole imaging range is thus scanned.
At that time, only one scan reference time is defined in one cardiac cycle. The mode of scanning is an axial scan, or a helical scan with low helical pitch, for example, a helical pitch of 0.2 or lower. Moreover, data collection by the scan on a heartbeat-by-heartbeat basis is achieved by employing a half scan technique that collects scan data in a view angle range of 180 degrees plus a radiation fan beam angle so that temporal resolution comes first is disclosed in Patent Document 1 (Japanese Patent Application KOKAI No. 2004-202246). The half scan technique is also referred to as segment scan technique or partial scan technique. These are reasonable conditions considering the facts that: an actual cardiac cycle, i.e., an R-R interval, is of the order of 0.75 to 1.2 seconds and a period of small change in which motion of the heart is small, for example, diastole, occupies around 25% of the cardiac cycle; the upper limit of the velocity of gantry rotation is currently of the order of 0.35 seconds/rotation; and extremely high-speed movement of a table on which the subject is laid is impractical.
Moreover, image reconstruction is performed for a unit based on the defined reference time. Specifically, from scan data collected by one heartbeat-based scan, image reconstruction only for a slice in a scan region at that time is performed. This is because the object to be imaged is a heart, which is an anatomical part moving relatively fast and in a complicated manner, and hence, respective periods of collection of data used in image reconstruction of several slices are aligned with one another as much as possible to prevent variation of the data from appearing on reconstructed images of the slices arranged in the body axis direction.
Furthermore, the scan region in the heartbeat-based scan is defined so that the regions for the temporally adjacent scans partially overlap one another. The reason thereof is as follows: in the case that a half scan is performed for the heartbeat-based scan, image reconstruction based on scan data therefrom results in insufficiency of projection data that lie substantially mutually opposite in a direction of a reconstruction plane of the slice (referred to hereinbelow as conjugate data), leading to generation of a missing cone region having no data in some region in the vicinity of either end in the slice direction (z-axis direction) of the detector during the scan, as shown in FIG. 10. Thus, noticeable cone-beam artifacts appear in reconstructed images of such slices in the vicinity of either end. Accordingly, from scan data of a half scan within one cardiac cycle, image reconstruction only for a region slightly inward of its scan region is performed. Specifically, as shown in FIG. 11, a scan region in the heartbeat-based scan is defined so that adjacent scan regions partially overlap one another. Thus, in conventional cardiac-gated imaging, adjacent scan regions are defined to partially overlap one another in order to reduce disadvantages from application of a half scan.
Nevertheless, there is still found a missing cone in the vicinity of either end for each scan region depending upon the conditions, leading to generation of cone-beam artifacts in reconstructed images of slices in the vicinity of either end of the region. Therefore, when a three-dimensional image is produced by stacking in the body axis direction reconstructed images of slices lying at positions in the body axis direction, boundary artifacts occur in the vicinity of the border between the scan regions.
One method for solving this problem is a method involving correcting an image in the vicinity of a border between adjacent scan regions by referring to reconstructed images of nearby slices which is disclosed in Patent Document 2 (Japanese Patent Application KOKAI No. 2001-076169), or producing these reconstructed images by simply or adaptively combining them so that discontinuity of the images is thereby reduced. For example, an image in the vicinity of a border between scan regions is produced by applying weighted addition to a reconstructed image of a slice closer to one side of the vicinity and that of a slice closer to the other in a proportion of 1:1.